ABSTRACT Vocalization acoustic features can serve as a potent biomarker of physiological stress and signal a person?s physiological state. However, few studies have explored the relation of vocal stress as part of the co-regulation of infant-parent interactions. The proposed R03 study attempts to fill this gap by applying neuro-physiologically informed vocalization stress measures to assess the role that mother?s physiological stress and concurrent alterations in vocal acoustic features play in the co-regulation of mother-infant interactions. Drawing from the theoretical frameworks of the Mutual Regulation Model (MRM) and the Polyvagal Theory, this study will use audio data obtained from 1 R01 HD083267; E.T. coPI; ?parent grant?, a study using an experimentally induced acute stress to evaluate the effects of an acute stress on maternal interactive behavior and mother-infant interactive co-regulation. Specifically, the methods in the parent project are: To experimentally manipulate maternal exposure to stress and to study the stressors? cascading effects on the organization of the dyadic- interactive regulatory system in infants and their mothers. Dyads will be randomly assigned to one of two experimental conditions: an experimental maternal stress (listening to infant distress cries) or non-stress condition (listening to infant positive vocalizations). Following a resting physiological baseline, all dyads begin with a face-to-face play episode to evaluate their pre-stress dyadic regulatory communicative behaviors and physiology. Next, mothers participate in the experimental maternal stress or non-stress condition, followed by a second face-to-face play episode. Individual and dyadic stress effects are evaluated using a unique combination of multiple measures that includes behavior, salivary hormones, cardiac, and electrodermal activity collected from mothers and infants, and self-reported stress recorded from mothers. The overall hypothesis of the parent grant is that the dyadic behavioral and physiologic regulation will be disrupted in the stress group compared to the non-stress group. In this R03 mother vocalization stress will be added as a non- contact objective biomarker of maternal stress to overcome limitations of current stress measures. The vocal stress features will be quantified by frequency band measures as well as pitch and rhythmic instabilities, which reflect the neurophysiological regulation of the laryngeal and pharyngeal muscles. Vocalization parameters will be assessed for stress-related changes caused by the experimental manipulation, relations with mother physiological and perceived stress, relations with dyadic co-regulation, and mediation analyses to examine whether experimental stress inductions can affect infant-mother dyadic co-regulation via changes in mother?s vocal acoustic features.